1. Field of the Invention
The present invention relates to an active noise control apparatus and a waveform transforming apparatus, and is effectively used in actively attenuating noises by outputting cancelling sounds from a speaker against the noises to combine the cancelling sounds and the noises.
2. Description of the Related Art
A conventional noise attenuating method is to attenuate noises by actively combining, in amplitude, generated sounds against the noises. For example, noises can be cancelling by generating sounds opposite in amplitude to the noises through an FIR filter, radiating the sounds from a speaker, and combining the noises with the sounds opposite to the noises in amplitude.
FIG. 1 is a block diagram showing the configuration of such a conventional active noise control apparatus.
In FIG. 1, a noise control filter 301 comprises an FIR filter. A noise control object system 302 is a system to be noise-controlled. That is, system 302 indicates the transmission characteristics in space from the speaker connected to the noise control filter 301 to a microphone in an acoustic coupling system 303. An update unit 304 updates the filter coefficient of the noise control filter 301. An error pass filter 305 comprises an FIR filter. An operation unit 306 performs an operation using the method of least mean squares.
The filter coefficient of the error pass filter 305 is preliminarily defined according to the impulse response from the noise control object system 302.
For example, an input signal X.sub.(n), which is a primary noise generated by, for example, a fan, is input to the noise control filter 301, and the input signal X.sub.(n) is filtered through the noise control filter 301 and output as an output signal Y.sub.(n). The output signal Y.sub.(n) is output through a speaker as a secondary noise to the noise control object system 302 which is, for example, a space coupled to the fan by a duct (not shown) in FIG. 1. The secondary noise output to the noise control object system 302 is transmitted through the noise control object system 302, becomes a cancelling sound d.sub.(n) ', and combines with the sound d.sub.(n) that is the primary noise transmitted through the duct. The composite sound is detected by the microphone, thereby generating an error signal e.sub.(n) =d.sub.(n) -d.sub.(n) '.
The error signal e.sub.(n) is provided for the update unit 304. The update unit 304 is also provided with an input signal X.sub.(n), and an acoustic coupling error signal is generated by passing the input signal X.sub.(n) through the error pass filter 305. The update unit 304 refers to a reference signal and determines through the operation unit 306 a filter coefficient of the noise control filter 301 in such a way that the root-mean-square value of the error signal e.sub.(n) can be minimized. n refers to a sample number.
However, assuming that the frequency of the sound to be cancelled ranges from 20 Hz to 1 kHz, the impulse response is 20 mS, and the time required for a 1-tap process of the FIR filter is 250 .mu.s, the number of taps of the FIR filter is 192 using the conventional active noise control apparatus. If an IIR filter replaces the FIR filter, then the acoustic feedback canceler indicates 224 taps, the adaptive filter indicates 80 taps, and the error pass filter indicates 96 taps. Thus, a total of 400 taps are required. Therefore, a large memory capacity and a long DSP operation time are required to cancel the noises using the cancelling sounds, thereby requiring expensive circuitry and high operation costs.